There have been used two types, beneath-light type and edge-light type surface light source elements for an image display apparatus.
The beneath-light type surface light source element includes a plate-like member provided with a light-emitting surface and a plurality of primary light sources disposed on a back surface of the plate-like member. This type has a characteristic that it is easy to be large in size because the light sources are disposed on the back surface opposite to the light-emitting surface, and the element is widely used as a display of a television having a liquid crystal display device. Generally, the plate-like member on which the light-emitting surface is provided is structured by a plurality of optical sheets referred to as diffusion plates, prism sheets, diffusion sheets.
On the other hand, the edge-light type surface-light source element has a characteristic that it is possible to have a thickness thinner than the beneath-light type surface light source element because a plurality of primary light sources are disposed on a side surface of a light-guiding plate, and the element is widely used as displays such as mobile notebook computers, monitors or the like. Light emitted from the primary light sources is entered a light-guiding plate made of a transparent plate of transparent polymer molecule such as PMMA (polymethylmethacrylate) or the like, bent, transmitted in the light-guiding plate and emitted from an exit surface which is one of two principle surfaces of the light-guiding plate toward a liquid crystal panel. An optical sheet referred to as a prism sheet, or diffusion sheet and configured to collect light emitted from the exit surface and accomplish a high brightness is also used.
A printing of white dots is applied onto a reflection surface which is one of the two principle surfaces of the transparent plate-like light guide (light-guiding plate) to improve light use efficiency, and a size and a density of the dots are adjusted so that brightness distribution in the light-emitting surface of the surface light source element in a direction of viewing is equalized. Similarly, there is used a light guide in which a pattern including dots each having 100 to 500 μm in size and a disc-like shape having 10 to 50 μm in thickness is formed, and a top surface of the pattern is roughened is directly provided on the reflection surface of the light-guiding plate.
There is proposed a light guide (Patent Documents 1 to 3) provided with a pattern having more high use efficiency compared with the printing type light guide.
For example, Patent Document 3 discloses a light guide including concave or convex portions each having a trapezoidal shape in section, provided on at least one of an exit surface and a bottom surface. By the light guide, light entered the light guide through an incident end surface is taken in the bottom surface, and reflected light thereon is configured to be directed to the exit surface. In addition, by emitting light from the exit surface through trapezoidal convex stripes provided on the exit surface by use of the light guide, it is possible to emit light entered the light guide in a direction perpendicular to the incident end surface with an angle near a front direction.    Patent Document 1: Japanese Patent Application Publication No. H10-282342    Patent Document 2: Japanese Patent Application Publication No. 2003-114432    Patent Document 3: International Application Publication No. WO2006/013969A1